Make erlang.mk manage the Erlang version required for a project

[essen]

For projects that depend on a single Erlang version (ie most private projects, unlike open source ones), allow erlang.mk to manage the Erlang version and use this version automatically when working on that project, so you could go from a project to another and have the right version used automatically.

[essen]

This already exists via the ERLANG_OTP variable.